<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
		<script type="text/javascript">
			// typeof()判断这个变量的类型
			var num;//只是声明，没有赋值，现在的值undefined
			console.log(num);//undefined
			console.log(typeof(num));//undefined
			num = 3.14;
			console.log(num);//3.14
			console.log(typeof(num));//number
			num = null;
			console.log(num);//null
			console.log(typeof(num));//object
			num = "JS";
			console.log(num);//JS
			console.log(typeof(num));//string
		</script>
	</head>
	<body>
	</body>
</html>
